In image-based eye tracking an image capturing device, such as a digital camera or an imaging sensor, acquires a digital image of the eye and processes the image to estimate gaze direction, pupil size or other eye-related features. Image processing techniques for eye tracking may be expensive in terms of computation time, power consumption, memory footprint, etc. Additionally, eye-related events may be comparably “fast” or “brief”. Hence for accurate eye tracking, image acquisition and image processing needs to be performed at a correspondingly high frame rate. Some applications of eye tracking require real time or near-real time performance as the eye tracking may be used as a user-interaction method, so low latency is important.